 Otto Jahn, in his 1856 Mozart biography, reported that Haydn and Mozart used the informal "du" forms in conversation, a practice unusual at the time for two people of such different ages, hence evidence for a close friendship.Jahn relied on the testimony of Mozart's sister-in-law Sophie Haibel as well as Haydn's friend and biographer Georg August Griesinger.Haydn last saw Mozart in the days before he departed for London in December 1790.